This is shaping up to be a classic example of the third of five rules of thumb posted here last week. Pat Toomey has had a solid lead, ranging from 5 to 10 points depending on the polling organization. That lead has been thinning in the recent polling data, as late-breakers and leaners start to reduce the number of undecided voters responding to pollsters questions.

However, history shows a fairly consistent trend—polls tend to tighten up between Columbus Day and Election Day, but as the last late-breakers make up their minds, they tend to break for the leader. Since the seat is currently being held by a Democrat, and since the Democrat running for the open seat has so far failed to garner a 50% level of support, some of the other rules of thumb may come into play as well. Specifically:

An incumbent with less than 50% among surveys of likely voters tends to be unseated. While Sestak isn’t the incumbent, he is a sitting member of Congress and the seat is currently being held by his party. It’s as close to incumbency as you’ll see in an open-seat race.

Late-breaking, undecided voters tend to break for the leader in tight races also.

What’s going to make this race interesting to watch is the fact that Sestak has something of a reputation as a comeback kid. The Morning Call story detailing the latest poll with Sestak pulling ahead has this:

It wouldn't be the first time Sestak surged with the finish line in sight. In two previous elections, when political pundits had counted him out, Sestak came from behind with just weeks to go and emerged victorious.

In May, the same tracking poll was the first to show Sestak catching and eventually surpassing U.S. Sen. Arlen Specter in the Democratic primary. A tracking poll shows who's ahead and by how much each day until the election.

Many experts credited the Delaware County congressman's victory over Specter to an electorate unhappy with the 30-year incumbent senator, not because they were overly impressed with the younger, upstart Navy admiral.

So, could the Pennsylvania Senate race be one that breaks the historical mold and truly produces a comeback finish? After all, didn’t Sestak do just that in the primary against Arlen Specter, the bona fide incumbent? Perhaps, but the primary race showed some of the characteristics you’d expect: A troubled incumbent failing to draw 50% of support, late-breakers going for the challenger and in at least some polling, late-breakers going for the leader as well.

Some new polling results are likely to hit the streets next week. We’ll see how this shapes up and compare the results on November 3 (or later, depending on how tight the race ends up being).

UPDATE: Nate Silver at Five Thirty Eight is unconvinced that a bona fide swing is occurring.